User Tools

Site Tools


Using shx/shy as stx,y/sty,x

The 6510 doesn't have a stx abs,y or sty abs,x. So instead you would normally do something like this:

//store X indexed by Y:
sta address,y

//store Y indexed by X:
sta address,x

Which each take 7 cycles. But instead you can do this, which only takes 5:

//store X indexed by Y:
shx address,y

//store Y indexed by X:
shy address,x

However, there's a little catch. The value is and'ed with the high byte of the address before it's stored. And to make it even more confusing, it's actually the high byte + 1. So if you store it in $fe00, the value is and'ed with $ff, and therefore unaffected. But since you don't always use all the 8 bits, other addresses might work as well. E.g. if your values are C64 colors, which are between $00 and $0f, the upper 4 bits don't matter. So in that case all pages ending in $e will work, i.e. $0e00, $1e00, etc.

The opcodes are classified as unstable, but this only affects the and'ing. So sometimes the original X/Y value is stored, and sometimes it's and'ed first. But if you use value/address combinations where the and'ing doesn't matter, the result is always the same.

base/use_shy_as_sty_abs_x.txt · Last modified: 2017-11-16 01:38 by cruzer